The invention relates generally to a method of making cadmium sulfide. In particular, the invention relates to method of making cadmium sulfide layers used, for example, in a photovoltaic cell or a solar cell.
Solar energy is abundant in many parts of the world year round. Unfortunately, the available solar energy is not generally used efficiently to produce electricity. For example, a typical solar cell achieves a conversion efficiency of less than about 20 percent. The cost of conventional photovoltaic cells, and electricity generated by these cells, is generally very high. Moreover, photovoltaic cells typically include multiple layers formed on a substrate, and thus solar cell manufacturing typically requires a significant number of processing steps. As a result, the high number of processing steps, layers, interfaces, and complexity increase the amount of time and money required to manufacture these cells. Researchers are continually striving to improve the efficiency and reduce the production costs of the photovoltaic (PV) cells.
A typical polycrystalline thin film PV cell has a very thin (less than 0.1 micron) layer called the “window” layer. The window layer's role is to form a heterojunction in combination with an absorber layer. The window layer desirably is thin enough and has a wide enough bandgap (2.4 eV or more) to let most available light through to the absorber layer. For CIGS and CdTe type solar cells, the most common material for the window layer is cadmium sulfide (CdS). Cadmium sulfide is a direct bandgap semiconductor and has many other applications, for example in light detectors and photoresistors.
Various methods have been used to deposit CdS thin films, for example chemical bath deposition, sol-gel, electrochemical deposition, thermal evaporation, sputtering, and spraying. However, most of these methods are prohibitively expensive and/or complicated and thus could not be used to produce thin films in large quantities for reasonable cost.
Accordingly, there remains a need for more efficient and less expensive methods of fabricating thin films of cadmium sulfide for large-scale production.